Gas cleaning devices are for example employed in order to clean blow-by gases of a combustion engine from oil or solid particles. Since these particles are generally denser than the gas they can be separated from the gas by centrifugal forces in a centrifugal separator. A centrifugal separator can for example be hydraulically driven by a propellant oil exiting a turbine wheel which has substantially tangentially directed outlet nozzles or by a propellant oil hitting a turbine wheel. The propellant oil can subsequently be collected in a collection chamber and be returned to an oil pan of a crankcase from where it may have been pumped to the turbine wheel before.
However, in gas cleaning devices of the above-mentioned type gas in the collection chamber is mixed with the propellant oil from the turbine wheel, so that, when the propellant oil flows down in the outlet conduit, gas bubbles are generated therein which form a larger gas accumulation in the outlet conduit. The more gas being collected in this gas accumulation, the higher the pressure will be therein. The high pressure in the gas accumulation in combination with the low pressure of the propellant oil prevailing in the collection chamber makes it difficult for the oil in the bottom of the collection chamber to flow out through the oil outlet and down through the outlet conduit back to the oil pan of the engine, since the weight of the oil column in the outlet conduit is insufficient for compensating for the pressure acting upwards from the gas accumulation. As a result, the collection chamber is filled up with oil which eventually slows down the turbine wheel therein and thus deteriorates the function of the cleaning apparatus.